Due to the fact that ethanol is a renewable fuel, and for other reasons as well, the use of ethanol and ethanol blends (i.e., ethanol and gasoline) continues to grow. For example, flexible fuel vehicles are known that are designed to run on gasoline as a fuel or a blend of up to 85% ethanol (E85). Properties of such fuels, such as its conductivity or dielectric constant, can be used to determine the concentration of ethanol (or other alternate fuel) in the gasoline/alternate fuel blend and can also be used to determine the amount of water mixed in with the fuel. Experimental data shows that the fuel dielectric constant is directly proportional to the ethanol concentration but relatively insensitive to water contamination, provided that the water concentration is below about 1% since the dielectric constant of water is around 80 at 25° C. (i.e., surveys show that the water concentration on most U.S. Flex fuel stations is below 1%). On the other hand, fuel conductivity is very sensitive to water concentration. For example, ethanol has a dielectric constant of around 24 at 25 degrees Celsius while gasoline has a dielectric constant of around 2 at the same temperature. Determining the properties of such fuels is important for operation of a motor vehicle since an engine controller or the like can use the information regarding the composition, quality, temperature and other properties of the fuel to adjust air/fuel ratio, ignition timing and injection timing, among other things. Additionally, increasingly strict emissions-compliance requirements have only further strengthened the need for an accurate flexible fuel sensor.
As added background, most sensor technologies for fuel property sensing require in-situ signal processing electronics to convert the relatively small sensing signals to a suitably strong electrical signal that can be used by an external circuit, such as an engine controller, to define the measured fuel property of interest. For example only, a capacitive sensor, which is configured to apply an excitation signal to spaced apart sensing plates, induces a relatively small response signal, thus requiring local electronics to preserve the signal-to-noise ratio.
It is also known that most in-situ sensors (e.g., capacitive, inductive or magnetic technologies) do not require direct contact or exposure to the fuel in order to assess the relevant fuel properties. Nonetheless, these sensors generally benefit from the physical isolation from the fuel, since contact with the fuel can often degrade the performance of the sensor. While it is known to use coatings to isolate various sensor components from contact with the fuel, such coatings may induce stress and/or degrade the signal-to-noise ratio of the sensing approach.
Fuel passage obstruction is another shortcoming of conventional fuel sensors, particularly capacitance-based approaches. More specifically, to measure the capacitance of the fuel, conventional sensors are known to use plates with different shapes, but in all such applications these plates are inside the fuel line (i.e., the fuel passage). This makes the construction of such sensors more complex and poses a potential for obstructing the fuel flow. Additionally, this approach imposes stricter requirements to protect the plates from corrosion by the ethanol, as described above.
There is therefore a need for a fuel sensor that minimizes or eliminates one or more of the problems set forth above.